The present invention relates to a motor driving apparatus which drives a motor by converting polyphase AC voltages into ones having a desired magnitude and frequency by means of an AC/AC direct converter such as a matrix converter. In particular, the invention relates to a motor driving apparatus which enables continuous operation of a power converter while preventing its overheat or damage of a motor or the power converter by restricting the amplitude of the motor currents.
In power converters for driving a motor as typified by inverters and matrix converters, to prevent overheat or damage of the motor or the power converter, a protection device is provided which monitors the amplitude of the motor currents and stops operation of the power converter by turning off all semiconductor switching devices constituting the protection device when the motor currents have exceeded a limit level.
However, a sudden stop of operation of the power converter causes sharp variations in the voltages applied to the motor and may damage the motor or driving devices connected thereto. Furthermore, this kind of control is not suitable for use in the power converter that the continuous operation is desired.
JP-A-2004-180390 (paragraphs 0015-0018, FIG. 1, etc.) discloses a conventional technique as a countermeasure against the above problems in which a restriction level lower than the limit level is set. When the motor currents have exceeded the restriction level, the power converter is kept in operation while further increase in the motor currents is prevented.
FIG. 5 is a block diagram of an important part of a conventional apparatus disclosed in JP-A-2004-180390. In FIG. 5, reference numerals 102-104 denote a power source, a power converter, and a motor, respectively. Output voltage instruction calculating means 100 calculates output voltage instruction values for the power converter 103. On the basis of the output voltage instruction values, PWM pattern generating means 101 calculates PWM patterns according to which the semiconductor switching devices of the power converter 103 are turned on/off.
On the other hand, current restricting means 108 receives power source voltages detected by the power source voltage detecting means 105 and motor currents detected by motor current detecting means 106, and calculates and outputs PWM patterns to be used for generating output voltage vectors which are opposite in directions to current vectors calculated from the motor current detection values.
Reference numeral 107 denotes comparing means in which a restriction level lower than a motor current limit level is set. The comparing means 107 compares the motor current detection values with the restriction level.
If the motor currents have exceeded the restriction level, switching means 109 is switched to the current restricting means 108 side in accordance with the output of the comparing means 107, whereby PWM patterns for generating output voltage vectors that are opposite in directions to current vectors are supplied to the power converter 103 via the switching means 109.
With the above measure, the motor currents decrease and their amplitude can be made lower than or equal to the restriction level. As a result, overcurrent can be prevented from flowing through the motor 104 while the power converter 103 is kept in operation.
In connection with the conventional technique of the above-mentioned JP-A-2004-180390, Japanese Patent Application No. 2005-238593, which was filed earlier than this application and was not laid open as of the filing date of this application, discloses an invention in which to reduce output current ripples by lowering the calculation load, a power converter is caused to output zero voltages.
FIG. 6 is a block diagram of an important part of an apparatus according to the above earlier-filed invention. In FIG. 6, components having the same functions as corresponding components shown in FIG. 5 are given the same reference numerals.
The apparatus of FIG. 6 is different from that of FIG. 5 in that zero voltage generating means 110 is provided in place of the current restricting means 108. The zero voltage generating means 110 selects and outputs, on the basis of the power source voltages, PWM patterns such that all the output phase voltages are made identical.
In the above configuration, when the motor currents have become higher than the restriction level, PWM patterns are supplied from the zero voltage generating means 110 to the power converter 103 via the switching means 109. For example, the power converter 103 outputs a maximum phase voltage of the power source 102 for all the output phases. The line voltages become zero when the voltages of all the output phases are the same. Therefore, the motor currents decrease and their amplitude does not exceed the restriction level.
The conventional technique of JP-A-2004-180390 shown in FIG. 5 provides a great current reducing effect because voltage vectors which are opposite in directions to motor current vectors are output. However, large ripples appear in the output currents because the output voltages of the power converter 103 are changed sharply.
Where the power converter 103 is an AC/AC direct converter such as a matrix converter, the input currents are also distorted if ripples exist in the output currents because the power source 102 is directly connected to the motor 104 by bidirectional semiconductor switches. Ripples in the output currents are not preferable because they may cause a torque ripple or noise in the motor 104. And ripples in the input currents are not preferable either because they may cause erroneous operations or the like in other apparatus which are connected to the power source 102.
On the other hand, in the earlier-filed invention shown in FIG. 6, the motor currents may increase rather than decrease under such operation conditions that the energy is returned from the motor 104 to the power converter 103 (regeneration) as in the case of a braking operation.
The reason why the motor currents increase if the power converter 103 output zero voltages during a regenerative operation will be described below.
FIG. 7 is a one-output-phase circuit diagram of a matrix converter in a case that a synchronous motor as the motor 104 is driven by a matrix converter as the power converter 103 of FIG. 6. The motor current during a driving operation is given by the following Equation (1):
                              i          o                =                              1                                          R                1                            +                              jω                ⁢                                                                  ⁢                                  L                  σ                                                              ⁢                      (                                          v                o                            -                              e                m                                      )                                              (        1        )            where vo is an output voltage of the matrix converter, em is an induction voltage generated by the motor, R1 is a primary winding resistance of the motor, Lσ is a primary synchronous inductance, and ω is a primary angular frequency.
On the other hand, the motor current during a regenerative braking operation is given by the following Equation (2):
                              i          o                =                              1                                          R                1                            +                              jω                ⁢                                                                  ⁢                                  L                  σ                                                              ⁢                      (                                          e                m                            -                              v                o                                      )                                              (        2        )            
It is seen that the signs of the output voltage vo and the induction voltage em in Equation (2) are opposite to those in Equation (1). In the configuration of FIG. 6, suddenly changing the output voltage of the matrix converter (power converter 103) to zero means that vo is made equal to “0” in Equation (1) (driving operation). The motor current io is thereby decreased. However, if vo is made equal to “0” in Equation (2) (regenerative braking operation), the value (em−vo) on the right side becomes larger than that before vo is made equal to “0” and hence the motor current io increases contrary to the intention. Referring to the circuit diagram of FIG. 7, when vo is made equal to “0”, short-circuiting is caused for the induction voltage em in the matrix converter and the current io is thereby increased.
A current restriction method of a voltage-type inverter having an energy buffer is known in JP-A-3-74175 (Japanese Patent No. 2,745,691). This document discloses a technique that when the magnitude of a current vector has become larger than a restriction value, a voltage vector that is closest in position to a vector that is opposite in direction to the current vector is selected and the voltage-type inverter is caused to output.
However, in direct converters such as matrix converters, the PWM pulse generation method is different from the method in inverters. Therefore, it is difficult to apply the above conventional technique to direct converters as it is.
An object of the present invention is therefore to provide a motor driving apparatus which drives a motor by means of an AC/AC direct converter such as a matrix converter and which does not cause ripples even in the event of sharp variations in the motor currents, can reduce the motor currents in each of a driving operation and a braking (regenerative braking) operation, and can keep safe operation of the direct converter.
Further objects and advantages of the invention will be apparent from the following description of the invention.